Well, given the one example event, one might try
... | rex "getTaskHistoryList (?<field_name>\d+)$"
However, a more thorough regex might be:
... | rex "Message: Time take for (?<operation>[^\s]+) (?<time_taken>\d+)$"
These are not particularly complicated regular expressions. If you are not already familiar, I would recommend studying how regular expressions work in general - there is a good website, http://www.regular-expressions.info/, and O'Reilly has an excellent (if a little aged) paperback book on the subject, http://shop.oreilly.com/product/9780596528126.do
Also, you should study up on how Splunk uses regular expressions for field extraction. http://docs.splunk.com/Documentation/Splunk/4.3/Knowledge/Aboutfields is as good of a place as any to start.
